Sheet Metal Gauge Table

Imperial gauge to standard metric (S.I. units): 

Imperial Gauge	Imperial Equivalent in mm	Standard Metric Sheet in mm
10	3.25	3
12	2.64	2.5
14	2.03	2
16	1.63	1.5
18	1.22	1.2
20	0.91	0.9
22	0.71	0.7
24	0.56	0.6
26	0.46	0.5

When designing AHU's, we have to design the casing. A standard double sheet casing use 18-22 gauge sheets. The lower the gauge, the thicker the sheet. Thicker sheet improves sound reduction/deadening, as the sheet may absorb more of the vibration.